Audible has for its Premium Plus Members: Select Audible Books on sale at 2 Books for 1 Credit.

Offer only valid on Audible Books located in this offer link and must be an Audible Member to be eligible for offer
To redeem this offer, add two audiobooks to the cart from the selection provided. When you check out, you will be able to purchase both audiobooks for one credit – either by using a credit you have, or by purchasing one at the time of check out.